Add OAS contract monitoring support (#325)

* Add OAS contract monitoring support

* Pass the contract failure reason to UI

* Fix the issues related to contract validation

* Fix rest of the issues in the UI

* Add documentation related to contract monitoring feature

* Fix a typo in the docs

* Unmarshal to `HTTPRequestResponsePair` only if the OAS validation is enabled

* Fix an issue caused by the merge commit

* Slightly change the logic in the `validateOAS` method

Change the `contractText` value to `No Breaches` or `Breach` and make the text `white-space: nowrap`.

* Retrieve and display the failure reason for both request and response

Also display the content of the contract/OAS file in the UI.

* Display the OAS under `CONTRACT` tab with syntax highlighting

Also fix the styling in the entry feed.

* Remove `EnforcePolicyFileDeprecated` constant

* Log the other errors as well

* Get context from caller instead

* Define a type for the contract status and make its values enum-like

* Remove an unnecessary `if` statement

* Validate OAS in the CLI before passing it to Agent

* Get rid of the `github.com/ghodss/yaml` dependency in `loadOAS` by using `LoadFromData`

* Fix an artifact from the merge conflict

agent/pkg/api/contract_validation.go

@@ -0,0 +1,110 @@
+package api
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+
+	"github.com/getkin/kin-openapi/openapi3"
+	"github.com/getkin/kin-openapi/openapi3filter"
+	"github.com/getkin/kin-openapi/routers"
+	legacyrouter "github.com/getkin/kin-openapi/routers/legacy"
+	"github.com/romana/rlog"
+
+	"github.com/up9inc/mizu/shared"
+	"github.com/up9inc/mizu/tap/api"
+)
+
+const (
+	ContractNotApplicable api.ContractStatus = 0
+	ContractPassed        api.ContractStatus = 1
+	ContractFailed        api.ContractStatus = 2
+)
+
+func loadOAS(ctx context.Context) (doc *openapi3.T, contractContent string, router routers.Router, err error) {
+	path := fmt.Sprintf("%s/%s", shared.RulePolicyPath, shared.ContractFileName)
+	bytes, err := ioutil.ReadFile(path)
+	if err != nil {
+		rlog.Error(err.Error())
+		return
+	}
+	contractContent = string(bytes)
+	loader := &openapi3.Loader{Context: ctx}
+	doc, _ = loader.LoadFromData(bytes)
+	err = doc.Validate(ctx)
+	if err != nil {
+		rlog.Error(err.Error())
+		return
+	}
+	router, _ = legacyrouter.NewRouter(doc)
+	return
+}
+
+func validateOAS(ctx context.Context, doc *openapi3.T, router routers.Router, req *http.Request, res *http.Response) (isValid bool, reqErr error, resErr error) {
+	isValid = true
+	reqErr = nil
+	resErr = nil
+
+	// Find route
+	route, pathParams, err := router.FindRoute(req)
+	if err != nil {
+		return
+	}
+
+	// Validate request
+	requestValidationInput := &openapi3filter.RequestValidationInput{
+		Request:    req,
+		PathParams: pathParams,
+		Route:      route,
+	}
+	if reqErr = openapi3filter.ValidateRequest(ctx, requestValidationInput); reqErr != nil {
+		isValid = false
+	}
+
+	responseValidationInput := &openapi3filter.ResponseValidationInput{
+		RequestValidationInput: requestValidationInput,
+		Status:                 res.StatusCode,
+		Header:                 res.Header,
+	}
+
+	if res.Body != nil {
+		body, _ := ioutil.ReadAll(res.Body)
+		res.Body = ioutil.NopCloser(bytes.NewBuffer(body))
+		responseValidationInput.SetBodyBytes(body)
+	}
+
+	// Validate response.
+	if resErr = openapi3filter.ValidateResponse(ctx, responseValidationInput); resErr != nil {
+		isValid = false
+	}
+
+	return
+}
+
+func handleOAS(ctx context.Context, doc *openapi3.T, router routers.Router, req *http.Request, res *http.Response, contractContent string) (contract api.Contract) {
+	contract = api.Contract{
+		Content: contractContent,
+		Status:  ContractNotApplicable,
+	}
+
+	isValid, reqErr, resErr := validateOAS(ctx, doc, router, req, res)
+	if isValid {
+		contract.Status = ContractPassed
+	} else {
+		contract.Status = ContractFailed
+		if reqErr != nil {
+			contract.RequestReason = reqErr.Error()
+		} else {
+			contract.RequestReason = ""
+		}
+		if resErr != nil {
+			contract.ResponseReason = resErr.Error()
+		} else {
+			contract.ResponseReason = ""
+		}
+	}
+
+	return
+}